This is a set of optional 0.9mm clutch springs. The stiffer the spring, the later the clutch will engage allowing the engine to spool up to a higher RPM range before the clutch comes on. The later the clutch engages, the more the clutch will slip causing the clutch shoes to wear out more quickly. With these softer clutch springs the engine will engage earlier, which can be useful if the traction is very low.